A little bit of background on *NIX printing. All *NIX applications output postscript.
Its a format independent page description language. This means its up to the receiver, it need not be a printer, to render the page onto the display medium.
If your printer understands postscript, you are mostly good, unfortunately postscript has versions.

For printers that don't understand postscript, or have some other wierdness, the postscript that you would like to print has to be rendered before its sent to the printer.
CUPS and ghostscript between them do this. Well ghostscript does the rendering.

Your git clone is a source code repository. You need to ./configure, make and install the software. Ideally, you would write an ebuild but you can do in by hand.
Don't do any more that you must as root.
Do read the installer script. If it installs to /usr/local, or even /opt, thats harmless. Portage won't notice it there.

Let me play with the build system for a while. I dan't have a DCP 7030 printer, so I cant test.

-- edit--

The site is down for me :(_________________Regards,

NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.